1. Field of the Invention
The present invention relates to a radio frequency (RF) signal receiver, and more particularly, to a direct conversion RF signal receiver.
2. Description of the Related Art
A direct conversion method is a method by which a signal in an RF band is down-converted into a baseband signal by mixing once the RF signal with a local oscillation signal. That is, the RF band signal is directly converted into a baseband signal, not converted into an intermediate frequency (IF) band signal and then into a baseband signal. In general, the former is referred to as a heterodyne method, while the latter is referred to as a direct conversion method.
FIG. 7 is a diagram of a conventional direct conversion receiver (hereinafter referred to as a DCR). When the conventional DCR receives an RF signal, a low noise amplifier 110 amplifies the RF signal and a transformer 120 converts the amplified RF signal into a differential signal. The differential signal is converted into an in-phase differential signal (I_WRF) and a quadrature-phase differential signal (Q_WRF) through a poly-phase filter 130 and input to a down-conversion mixer unit 710. The mixer unit 710 comprises a first mixer 711 and a second mixer 712 that mix the in-phase differential signal (I_WRF) with a first local oscillation signal (OS1) and a second local oscillation signal (OS2), respectively; a third mixer 713 and a fourth mixer 714 that mix the quadrature-phase differential signal (Q_WRF) with the first and second oscillation signals (OS1, OS2), respectively; and filters 721 through 724 that low pass filter the outputs of the respective mixers 711 through 714. Here, the second local oscillation signal (OS2) has the same oscillation frequency as that of the first local oscillation signal (OS1), but a 90° phase difference from the first local oscillation signal (OS1).
A subtracter subtracts the output signal (QQ) of the fourth mixer 714 from the output signal (II) of the first mixer 711 to output an I-path signal (I_PATH). An adder adds the output signal (IQ) of the second mixer 712 and the output signal (QI) of the third mixer 713 to output a Q-path signal (Q_PATH). The I-path signal (I_PATH) and Q-path signal (Q_PATH) are baseband signals converted from an RF signal.
The DCR having the structure shown in FIG. 7 usually has a phase and gain mismatch. The phase and gain mismatch in the DCR occurs in the poly-phase filter 130 and the mixer unit 710. Ideally the phase difference of the in-phase differential signal (I_WRF) and the quadrature-phase differential signal (Q_WRF) output from the poly-phase filter 130 is 90°, but the actual phase difference is not. Also, ideally, the phase difference of the first and second local oscillation signals (OS1, OS2) provided to the mixers 711 through 714 is 90° and the gains of the output signals of the mixers 711 through 714 are the same. However, the phase difference of the first and second location oscillation signals (OS1, OS2) is actually 90±φ (causing a phase mismatch. Also, a gain mismatch occurs in the output signals of the mixers.
If the phase and gain mismatch occurs in the DCR, as described above, the error rate of the received signal increases due to the mismatch, or the signal fidelity is degraded. Accordingly, to prevent distortion of a signal and to obtain a desired signal, it is important to identify the degree of phase and gain mismatch degree in the DCR and to calibrate for the mismatch.
However, to solve the phase mismatch the conventional methods have focused on a local oscillator, which generates a local oscillation signal. That is, most efforts involve generating a local oscillation signal without a phase mismatch. However, according to conventional methods, implementation of a local oscillator becomes difficult or the cost of implementation increases, resulting in a limitation to removing the mismatch.